Short take-off and vertical landing ("STOVL") military aircraft, also known as vertical and/or short takeoff and landing ("V/STOL", "VTOL", or "STOL") aircraft (hereinafter all of which are referred to as a "STOVL" aircraft for convenience), are used when a single aircraft is needed to attain both horizontal and vertical flight modes. A well-known example is the AV-8B "Harrier" type of STOVL aircraft. The vertical flight modes comprise aircraft takeoffs and landings from, e.g., aircraft carriers or other limited-length runways. Such aircraft generally include one or more conventional gas turbine engines that power the aircraft in both the horizontal and vertical flight modes.
To achieve the vertical thrust necessary for vertical flight modes, each engine on the STOVL aircraft may be coupled in some manner to one or more auxiliary lift fans. Depending upon their placement and orientation within the STOVL aircraft, the lift fans may also be used in conjunction with the associated engines to control the attitude of the aircraft (i.e., to control the aircraft pitch, roll, and/or yaw) during vertical flight. The lift fans are typically disposed within the aircraft fuselage and separate from the engine. The primary airflow axis of the lift fan is oriented vertically within the aircraft (i.e., the fan exhaust is pointed downward to generate vertical lift for thrust and control), while the primary airflow axis of the engine is oriented in the conventional horizontal direction. However, the engine exhaust is typically adjustable in a well-known manner from a horizontal position for normal horizontal flight to a vertical position for vertical flight modes. This way, the direction of the thrust produced by the engine may also be varied between horizontal and vertical.
The lift fan may be selectively aerodynamically coupled to the fan exhaust or turbine exhaust of the engine, or the lift fan may be selectively mechanically coupled to the fan or low rotor spool of the engine by a rotating drive shaft. In the latter case, a clutch and gearbox mechanism is usually employed to selectively engage and disengage the lift fan with the gas turbine engine. Examples of these types of well-known propulsion systems for STOVL aircraft are given in U.S. Pat. Nos. 5,464,175, 5,312,069, 5,275,356, 5,209,428, which are all hereby incorporated herein by reference.
Generally, proper and stable attitude control of a STOVL aircraft during vertical flight modes is inherently difficult to achieve. The amount of force required to attain the desired aircraft attitude control, together with the reaction time of this force, is critical to proper aircraft control in vertical flight. Current, known attitude controls for STOVL aircraft burden the aircraft with extra weight. They also cause the engine to run at temperatures exceeding nominal operation.
For example, the attitude control system for the Harrier STOVL aircraft consists of additional and separate control system hardware. In the Harrier aircraft, that hardware weighs approximately 200 pounds and has no other function than to provide control power during vertical flight modes. The Harrier's attitude control system also has an undesired inherent coupling between overall system thrust and the use of control power for aircraft attitude. Further, these attitude control requirements significantly increase engine turbine temperatures in the Harrier aircraft to the extent that a water injection system is needed to meet turbine durability requirements. Clearly, the additional attitude control system for the Harrier aircraft poses undesired burdens on the aircraft, such as weight, cost and the risk e.g., of component failure.